In an isolated shed, the company behind the flaming cauldron is working on a
secret closing ceremony plan

We are chasing a photographer, who is busy snapping, around a vast, corrugated iron hangar on an old military airfield near Harrogate, North Yorkshire. “Stop!” yells Simon Wood, sales director at scenery manufacturer Stage One, suddenly. “You can’t take pictures of that – it’s top secret. Move away!”

All around us, blue sparks are flying as men in white overalls weld huge pieces of glowing metal on to a spherical frame. Paint is spattered across rows of worktops, and plywood scraps lie discarded on the floor. Along one wall, sulphuric acid sloshes from side to side in deep vats; fixed to another is a plaster cast of a human head, a model planet and what looks like a dinosaur skull.

For the past nine months, this hangar concealed one of the best kept secrets of London 2012: that spectacular Olympic cauldron, revealed to a global TV audience of billions at the opening ceremony just over a week a go. In a stunning display of design and engineering, 204 copper petals, one for each competing nation, were lit by seven young athletes and brought together to form a single, symbolic flame.

Since then, the phones haven’t stopped ringing at Stage One, responsible not only for the cauldron but for many of the the mechanical parts required, from the wires that suspended the Mary Poppinses to the dancing house that concealed Sir Tim Berners-Lee, inventor of the World Wide Web. But media interest aside, the pressure is still on as the staff are hard at work producing props for the closing ceremony and the Paralympic Games.

I am one of the first journalists to be given a tour of the former Ministry of Defence base transformed into a factory of Olympic magic. I feel like I’ve wandered on to the canvas of an abstract painting or a sci-fi film set. Scorched metal petals, left over from rehearsals, are neatly perched in numbered pigeonholes. The wings worn by the cyclists, made from polystyrene, feathers and white gauze, are propped up on a wire frame. Weird and wonderful creations in various stages of production, from purple pianos to a huge steel chassis, line the walls.

“We do a lot of exciting things here,” laughs Wood. “There’s always something being built or tested, and it can seem a bit 'mad scientist’.” In one corner of the warehouse are props for the Paralympics ceremonies. Wood doesn’t take his eyes off the Telegraph’s photographer. There are towering metal arches and a monstrous animal shape carved from a slab of plastic. Bulging, angular constructions are draped in dust sheets. It’s frustrating being so close to Olympic props and gadgets to be unveiled later this month and not to ask for details - but I am sworn to secrecy.

In the past few days, however, some clues about the closing ceremony on Sunday have emerged with photographs of what appears to be a mini London Eye – a 30ft-high ferris wheel – and replicas of Tower Bridge, Big Ben and St Paul’s Cathedral, all being constructed at a secret location in Barking, east London. Del Boy’s Reliant Robin from Only Fools and Horses and a black cab are thought to feature – but Locog, and the creative director, Kim Gavin, who masterminded Take That’s Circus tour, remain tight-lipped.

So what will the bosses at Stage One talk about? The cauldron? “It was a challenge,” admits Jim Tinsley, the group’s technical director. “I always said I’d never touch an Olympic cauldron with a barge pole; it’s like a poisoned chalice. The Athens one nearly went very wrong; Vancouver went wrong; Sydney went wrong. But we do a lot of stupid things – and this was just another stupid idea.”

The designer, Thomas Heatherwick, commissioned by Danny Boyle, the mastermind behind the opening ceremony to come up with a cauldron that would go down in Olympic history, has described Stage One’s headquarters as “James Bond’s gadget workshop” and “the most sophisticated shed in Yorkshire”. The 17-ton cauldron, measuring 8m across and 8.5m high, required 300 sheets of copper and a team of 50 working 15 hours a day, seven days a week. Workers hammered, chiselled and moulded the petals, steel rods and perfected the mechanical system. They were, of course, sworn to secrecy; had to use code words to describe the parts - the flame was 'Betty’ and the base was ''Frank’’ - and banned from telling even their families what they were doing.

Heatherwick, whose previous creations include the East Beach Café in Littlehampton and London’s new Routemaster bus, said: “We didn’t want just another honking great dish on a stick. We wanted to show the countries coming together, reflecting human excellence.”

It was last October that he turned to Stage One, famous for producing the colossal Cycladic head at the 2004 Athens Games and the Olympic rings that hang from Tower Bridge. “There’s nothing you could say that would faze them,” Heatherwick explains. “They are used to dealing in fantasy, so I had every confidence it would work.”

Heatherwick’s team spent 1,000 hours sketching the petals to make sure each was unique, then metalworkers built a series of prototypes. Hundeds of materials, from nickel to stainless steel and sprayed metal were tested, before settling on copper, which was hammered into shape by hand.

Each petal was cleaned in acid, polished and inscribed with the name of the country that would carry it during the Parade of Nations. With three batches to make – for rehearsals, the opening ceremony and the Paralympics – the petals took three months to produce. Then there were the rods attaching them to the base, made from stainless steel buffed with blue-black paint, and gas burner fittings imported from Australia.

“We’ve been working flat out,” says Matthew Kelly, 29, a welder and night-shift supervisor. “When we heard we were making the flame, we thought it was that little torch they carry through the streets. You only get glimpses here and there, so you can’t tell what the whole thing looks like. Seeing it now on television in the Stadium is surreal; it is amazing knowing I’d helped build that.”

The Olympic cauldron was smuggled down the M1 to London in June, under police escort, one quiet Sunday night. “We told locals it was the bell,” says project manager Neil Franklin. “To keep a surprise like that, you have to get good at lying.” One of the biggest risks, he says, was testing the structure outdoors. “We’re in the middle of nowhere, but our worry was planes flying overhead.”

“It was nerve-wracking when the flame was lit,” says Wood. “It would have been like an athlete not getting the gold if it hadn’t worked.” Tinsley, who was in the stadium for the opening ceremony, forgot to look at the cauldron because he was busy watching Boyle’s expression. “He was euphoric. Seeing his face, seeing how passionate he was; that made it all worthwhile.”

The next challenge is the closing ceremony, after which the cauldron will be dismantled. Each country will receive a petal as a memento, while any spares will be melted down and recycled to prevent them appearing on coffee tables or being sold on eBay.

“They’ll be kept in cabinets in boardrooms and offices, I imagine,” says Thomas Heatherwick. “It will be interesting to see which countries get the Brasso out and shine the petals up, and who’ll leave them blackened from the flame.”

As long as they can get the flame to go out, Tinsley jokes, the closing ceremony should be “a doddle”. But there will be no time to rest. The Paralympics opening ceremony is just a few weeks away and there’s a new batch of petals to finish, props to assemble and a warehouse full of gadgets to paint, polish and test before August 29.

Any hints about what to expect? “The petals will be involved but they’re going to be used in a different way,” reveals Heatherwick, mysteriously. Wood is giving nothing away. “All I can say is you’re in for another treat,” he grins.